Aircraft that are either parked on the ground or are on the ground between flights can accumulate snow, ice, freezing rain or frost on the aircraft surfaces in cold winter weather. The presence of such deposits, particularly on airfoil surfaces, is a generally unsafe airfoil condition in that it hampers and can thwart liftoff, or at least is highly undesirable during takeoff and early flight periods since even small accumulations can result in severe deterioration of the airfoil aero-dynamic performance characteristics, occasionally causing crashes on takeoff/liftoff resulting in loss of life.
Glycols have long been used in aqueous solutions of various strengths that are sprayed onto the aircraft, as a deicing agent, to remove snow, ice, freezing rain and frost deposits from aircraft surfaces. These glycol-based aircraft deicing fluids are usually applied heated to more effectively break the ice bond or to melt the snow. A typical aircraft deicing fluid is composed of an alkaline glycol (typically ethylene glycol, propylene glycol and/or diethylene glycol) and water (solvent) mixtures (roughly equal parts on a weight basis; the pH is about 7.3 to 9.0) that contains ionic corrosion inhibitors, flame retardants and surfactants that promote wetting of the fluid on the aircraft during spray application. Instead of glycol, these ADFs can optionally be formulated with glycerin in the same manner. As used hereinafter, it is intended that any reference to glycol-based ADFs or AAFs or UAFs will also include such fluids formulated with glycerin instead of, or in addition to, one or more glycols.
Providing the hot fluid is applied close to the aircraft's surface, using a heated fluid will minimize the amount required to remove the snow and ice. The deicing fluids can also be applied in a high-pressure stream to flush the snow and ice deposits from the aircraft's surface.
After this treatment, the ADF desirably remains as a film coating on the aircraft surfaces, to serve as an anti-icing agent that provides continued antifreeze protection and delays the further formation or accretion of snow, ice, freezing rain or frost deposits on the aircraft surfaces. The same AF, in various concentrations, may ideally be used for both deicing and anti-icing functions. However, without a thickening agent, the deicing fluids tend to run off the inclined surfaces of the wing and tail and the nearly vertical surfaces of the fuselage and tail fin. Therefore, without a thickening agent, the deicing fluids provide only limited anti-icing protection.
AFs for more effective anti-icing-protection, therefore, typically contain a thickening agent and desirably possess the following attributes (none of which precludes its use as a deicer):
(i) formation of an essentially continuous film coating, after its application by conventional spraying devices, even on non-horizontal aircraft surfaces critical to the aircraft's aero-dynamic performance during takeoff/liftoff such as the vertical tail fin, but not interferring with the smoothness of the fuselage surfaces; PA1 (ii) extended protective anti-icing action; and PA1 (iii) viscosity and rheology characteristics that promote formation of an effective tenacious protective film) coating, yet enable the fluid coating to flow off the aircraft airfoil surfaces as a result of wind-shear forces during takeoff, prior to aircraft rotation. PA1 (A) about 1-99.9, preferably about 10-70, weight percent of one or more alpha, beta-mono- ethylenically unsaturated carboxylic acids, typically methacrylic acid; PA1 (B) about 0-98.9, preferably about 30-85, weight percent of one or more monoethylenically unsaturated monomers, typically ethyl acrylate; PA1 (C) about 0.1-99, preferably about 5-60, weight percent of one or more monoethylenically unsaturated macromonomers; and PA1 (D) about 0-20, preferably about 0-10, weight percent or greater of one or more polyethylenically unsaturated monomers, typically trimethylol propane triacrylate. PA1 (A) about 1-99.9% of one or more alpha, beta-monoethylenically unsaturated carboxylic acids; PA1 (B) 0 to about 98.9% of one or more monoethylenically unsaturated monomers; PA1 (C) about 0.1-99% of one or more monoethylenically unsaturated monomers, preferably macromonomers, containing at least one pendant hydrophobe moiety, preferably a complex hydrophobe moiety; and PA1 (D) 0 to about 20% of one or more polyethylenically unsaturated monomers, PA1 (1) at least about 40% of one or more glycols or glycerine or mixture thereof, PA1 (2) at least about 0.05% of the thickener, PA1 (3) a hydroxide, preferably a monovalent hydroxide, more preferably an alkali metal hydroxide, and preferably in the absence of amines, in an amount sufficient to provide a pH of at least about 7, PA1 (4) a surfactant which associates with the thickener, in sufficient amount to increase the thickening effect of the thickener, PA1 (5) optionally an effective amount of a corrosion inhibitor PA1 (6) optionally one or more dyes, and PA1 (7) the remainder being water. PA1 each R.sup.2 is the same or different and is a substituted or unsubstituted divalent hydrocarbon residue; PA1 R.sup.3 is a substituted or unsubstituted divalent hydrocarbon residue; PA1 R.sup.4, R.sup.5 and R.sup.6 are the same or different and are hydrogen or a substituted or unsubstituted monovalent hydrocarbon residue; and PA1 z is a value of 0 or greater. PA1 (i) the structure and concentration of the associative macromonomer, including: a) the size and structure of the hydrophobe; b) the moles of alkoxylation between hydrophobe and the double bond; c) the chemical nature of the bond between the alkoxylated portion and the reactive double bond (e.g., ester, ether, or urethane linkages); and d) the structure of the double bond itself (acrylic, methacrylic, crotonic, styrenic, etc.); PA1 (ii) the structure and concentration of acid monomer in the polymer (e.g., acrylic, methacrylic, crotonic, itaconic, etc.); PA1 (iii) the structure and concentration of the non-associative monomer, including monomers that cross-link the polymer during polymerization (e.g., trimethylol propane triacrylate), and those that leave cross-linkable functionality in the associative polymer without crosslinking during polymerization (e.g., 2-hydroxyethylacrylate); and PA1 (iv) the molecular weight of the polymer, as controlled by mercaptans during polymerization.
AAFs of the prior art are thickened typically with very large molecules (e.g., xanthan gum or various organic polymers, such as certain acrylics) which thicken by molecular entanglement and intermolecular friction. Such thickeners are deficient in that they do not provide optimal non-Newtonian behavior (i.e., their viscosity may not decrease sufficiently rapidly and/or extensively under wind-shear forces) for use under all weather conditions and on relatively slow-moving turboprop aircraft, such as commuter aircraft. Moreover, such thickeners are subject to undue degradation of viscosity as a result of severe shear forces imposed by the spraying nozzles used to apply an AF to the aircraft.
Important improvements in AAF technology were provided by compositions disclosed and claimed in U.S. Pat. No. 5,461,100 and copending, co-assigned U.S. patent application Ser. No. 08/065,237, filed May 20, 1993, U.S. Pat. No. 5,681,882 both of which are incorporated herein by reference. Those inventions were based on the unexpected discovery that certain macromonomer-containing polymeric thickeners which thicken by association among hydrophobes possess particular efficacy as thickeners for glycol-based aircraft anti-icing fluids. These fluids provided anti-icing fluids with protection times significantly longer than those obtainable with previous AAFs but, like previous AAFs, they found limited use as deicing fluids because the protection time afforded tended to be decreased by heating for long periods of time at the temperatures encountered in deicing operations.
The present invention is based on the unexpected discovery that such anti-icing fluid formulations can be modified so as to significantly improve their thermal stability, making them much more useful as deicing (as well as anti-icing) fluids.